1. Field of the Invention
This invention relates to a structure for protecting and insulating frozen substrates such as the permafrost in the Artic and sub-Arctic regions which comprises a base layer having hydrophobic or water impermeable properties applied to the frozen substrate with successive layers of a synthetic plastic foam such as polyurethane foam foamed in place on the base layer, each of such layers being separated from the next succeeding layer by a vapor penetration resistant layer and a hydrophobic or water impermeable layer applied to the uppermost vapor penetrationresistant layer.
2. Prior Art
The regions of the Arctic and sub-Arctic zones wherein perpetually frozen ground starts a few feet below the surface and extends downwardly to great depths are generally referred to as the permafrost regions. This permafrost is permanently frozen ground and consists of mixtures of varying content of water, salt, sand and gravel. In many cases the ice content of the upper portions may exceed 90 per cent of the total volume. This obviously is susceptible, therefore, to thawing whenever ambient temperatures exceed the freezing point of water.
At the surface of the ground there is an active growth layer called the tundra which covers the permafrost. In the winter months the ground is frozen solidly to the very surface of the tundra, but in the spring and summer thawing occurs to varying depths ranging from a few inches to several feet in an area between the tundra mat and permafrost zone which is called the "active layer". If left undisturbed the spring and summer thaw converts the tundra and active layer into a soggy marsh and thus the solid frozen ground which during the winter months can support excessively heavy loads, in the summer months is virtually incapable of being traversed by wheeled vehicles. Moreover, if during the winter months vehicles traverse an area of the tundra to such an extent that its surface is rutted even to a relatively small degree these ruts in the summer may subside or erode and can become gullies which release torrential and damaging run-offs, thus potentially permanently damaging the underlain permafrost and additional tundra structure.
The most common method of constructing roads, airfields and similar structures proposed for the permafrost region comprises depositing gravel on top of the tundra to a depth such that the gravel provides an insulating layer below which the active layer region remains frozen even during the summer thaws thereby protecting the underlain permafrost and thus provides a solid base for the roadway or other similar structure. Ideally, this gravel layer is sufficiently thick such that the frozen condition will actually form or penetrate up into the lower portion of the roadway gravel itself. In order to provide this insulating and protective structure the gravel thickness generally ranges from about at least 3 feet in the northernmost areas of the Arctic wherein the ambient summer temperatures are relatively low, and permafrost moisture content is relatively high, to 8 feet or more in the southern parts of the Arctic or sub-Arctic regions wherein higher ambient spring and summer temperatures are encountered and for longer times. Often, it is necessary to transport the gravel for such structures considerable distances when obviously little is available locally in many regions. This may be exceedingly costly and, consequently proposals have been made heretofore for providing other means for insulating and protecting the frozen Arctic and sub-Arctic substrates utilizing various methods of construction.
One of these proposals is set forth in U.S. Pat. No. 3,279,334, to Quartararo. It is proposed in this patent to place a layer of fiberglass or polyurethane plastic over the frozen ground in the winter months and along the edges of this layer, sheet piling is forced into the ground to prevent drainage and to prevent thawed soil from penetrating the frozen ground under the insulation and thus weakening the base. A layer of gravel is placed above the layer of fiberglass or polyurethane and finally a concrete road surface is applied on top of the gravel. The mean ambient temperature and duration during the summer months of the year is determined and thereafter there is selected the required thickness of the insulating plastic from known tables of insulating materials which will prevent a temperature rise in the base to a surface thawing temperature during the summer months.
The present invention not only obviates the need for great thicknesses of gravel such as that described hereinbefore and in general use in the Arctic and sub-Arctic regions, but it also obviates the disadvantages and provides advantages over other prior art structures such as that of the Quartararo patent.
In the structure of the instant invention a hydrophobic barrier or water impermeable layer of material is applied directly to the substrate whether it be the permafrost or tundra. This layer is exceedingly important since it provides a base for foaming in place the first layer of polyurethane foam. If the chemicals for forming the polyurethane rigid foam are applied directly to a cold wet surface, they do not react or foam since warm temperatures are required for the foaming which would melt the frozen substrate to produce water, and since a preferential reaction occurs between the isocyanate and the water, the polyurethane polymer does not form, i.e. the reaction is "killed". The base layer therefore permits the forming of the first polyurethane foam layer to take place without hinderance. Moreover, since it is generally the purpose to provide a road which will have a life span of a number of years, it is reasonable to assume that in a period of several years there will be at least one year in which higher than average summer temperatures will be encountered for a longer than average period of time. By the same probability, there will be at least one year in which higher than average winter temperatures will occur. Furthermore, there may be periods where "warm" summers and winters occur consecutively. The consequence of these probabilities is that excessive heat may accumulate at the insulation-permafrost interface. Thus, even though reasonable safety factors have been provided in the insulation thickness, it is quite possible that at some time during the life of the road some thawing may occur in the substrate. In the absence of a hydrophobic base coating over the substrate water will enter the polyurethane foam and thus destroy its insulation properties and cause disintegration of the road due to thawing of the substrate underneath. The effect of annual freeze-thaw on the insulation will make it progressively susceptible to water uptake. The hydrophobic base material protects the foam against this eventuality. Since the polyurethane is foamed in place to provide a rigid foam insulation which will be described further the polyurethane foam will follow and conform to surface irregularities in the tundra or permafrost. Thus, it is unnecessary to prepare the tundra or permafrost by grading or leveling, which grading would cause undesirable damage to these surfaces. By employing successive layers of foam on top of the base layer several advantages are realized. As each layer of foam is applied in place a certain degree of foaming occurs and on top of each layer there will be formed a layer of unfoamed material, generally referred to as the "skin" layer. The foam is unicellular and thus has insulating properties while the skin layer although not serving as an insulator is vapor penetration resistant thereby retarding movement of vapor into or out of the foamed layer. Moreover, this skin provides a greatly added strength both with respect to compression and flexure, much in the same manner as the well-known example of a bundle of sticks having much greater strength than a single stick of the same diameter. It is recognized that heretofore polyurethane foams have been sprayed on to surfaces for insulation purposes in layers. This, however, has been for a different reason, namely, to build up a desired thickness of foam, and in such instances it is preferred to have the foam layer as thick as possible and only if this is insufficiently thick are additional layers applied. In the instant invention the thickness of each foam layer is a critical feature of the invention and is carefully controlled to provide the desired number of layers per inch of total foam as will be described.
In the above-mentioned patented structure, piling is forced into the ground at the edges of the road to provide a moisture barrier. This is contrary to the purpose of the instant invention wherein it is desired to protect and avoid damage to the tundra surface and/or permafrost layer. Driving piles into the permafrost causes permanent and many times irreparable damage and, in addition, creates an area where subsidence and possible erodism may occur and become progressively worse each year. In the instant invention, at the edges of the roadway the layers of foam are allowed to taper down in thickness on the barrier coat to form what becomes substantially layers of skin only. As has been set forth the uppermost vapor penetration resistant layer, i.e. skin layer is coated with a hydrophobic barrier or water impermeable layer. Thus at the edges of the road even though some edge thawing occurs so that they are in contact with thawing tundra, lateral penetration of water vapor and of heat is prevented so that the interior of the structure over the frozen substrate remains as a protective structure.
In the absence of the lower and upper hydrophobic barrier the insulation, as described in the prior art, layer would behave like a wick for moisture and its insulation properties would be quickly destroyed. Thus, fiberglass, for example, is undesirable since it is not unicellular and the slightest penetration of moisture causes a wicking action to occur, thus destroying its insulating properties.
The use of polyurethane foamed in place instead of foamed in the form of rigid boards or the like provides a high structural strength since the foam follows the contour of the underlying surface and thus avoids "bridging" which in the case of rigid boards is a potential source of breakdown. Finally in providing a hydrophobic or water impermeable barrier above the uppermost skin layer on the foam, water penetration from above is avoided and thus the insulation properties and strength of the structure is maintained. None of these advantages are shown in the prior art known.